1. Field of the Invention
The present invention relates to a unidirectional isolated multi-level DC-DC converter and control method thereof. Particularly, the present invention relates to the unidirectional isolated multi-level DC-DC converter and control method thereof for reducing the dimensions. More particularly, the present invention relates to the unidirectional isolated multi-level DC-DC converter and control method thereof for minimizing a ripple quantity of output voltages and output currents.
2. Description of the Related Art
Generally, a conventional isolated DC-DC converter has been widely used in various industrial fields. Even though the conventional isolated DC-DC converter has a specific advantageous of simple and easy control, it has several drawbacks in operation, including low efficiencies, high ripple quantities, high electromagnetic interference and requiring the use of a filter circuit with a large capacity. Conversely, a conventional multi-level DC-DC converter has several advantageous of relatively higher efficiencies, relatively lower electromagnetic interference and allowing the use of a filter circuit with a relatively lower capacity even though it has a drawback of requiring sophisticated control.
For example, FIG. 1 shows a schematic view of a conventional multi-level DC-DC converter in accordance with the prior art. Referring initially to FIG. 1, the multi-level DC-DC converter 1 includes a dual half-bridge inverter 11, a dual high-frequency transformer 12, a full-bridge rectifier 13 and an output filter circuit 14 which are combined to form the multi-level DC-DC converter 1. In addition, the dual half-bridge inverter 11 must be constructed with four power switches and four capacitors.
With continued reference to FIG. 1, the dual half-bridge inverter 11 is formed from two separate half-bridge inverters serially connected. The dual high-frequency transformer 12 includes two high-frequency transformers each of which connects with an AC end of each half-bridge inverter of the dual half-bridge inverter 11. In power conversion operation, the power switches of the dual half-bridge inverter 11 are controllably switched such that the dual high-frequency transformer 12 can generate three levels of voltages at its primary side. Furthermore, the full-bridge rectifier 13 is utilized to rectify the three levels of voltages to thereby generate two voltage levels at a secondary side of the dual high-frequency transformer 12.
Disadvantageously, the two half-bridge inverters of the dual half-bridge inverter 11 require a sophisticated control manner and an arrangement of the four capacitors. However, the four capacitors may possess four different capacitances which may result in different voltages of the four capacitors and requiring more sophisticated control manner. In addition, the two high-frequency transformers of the dual high-frequency transformer 12 require using two iron cores which will result in increasing of manufacturing cost and dimensions.
However, there is a need of improving the conventional multi-level DC-DC converter and conversion method thereof. The above-mentioned prior art is incorporated herein by reference for purposes including, but not limited to, indicating the background of the present invention and illustrating the situation of the art.
As is described in greater detail below, the present invention provides a unidirectional isolated multi-level DC-DC converter and control method thereof. The unidirectional isolated multi-level DC-DC converter is formed with an inverter, a 3-winding high-frequency transformer, a first full-bridge rectifier, a second full-bridge rectifier, a selection circuit and a filter circuit. Only one of inverter and one of high-frequency transformer is provided to simplify a structure of the unidirectional isolated multi-level DC-DC converter. The selection circuit is provided to generate a low-harmonic pulse voltage to the filter circuit to thereby allow using a low capacity of the filter circuit in such a way as to overcome the problems of the conventional multi-level DC-DC converter and conversion method thereof multi-level DC-DC converter and conversion method thereof.